Russian gas exports via Ukraine to Europe finally dried up today, threatening millions of households in Eastern and Central Europe with hardship during a bitter cold spell and prompting more furious protests from Brussels.

Negotiations on a dispute over the price Ukraine has to pay for its gas supplies are due to resume in Moscow tomorrow and senior EU leaders tried their best to ratchet up the pressure.

José Manuel Barroso, President of the European Commission, urged the prompt restoration of supplies in telephone calls to Vladimir Putin, the Russian Prime Minister, and his Ukrainian counterpart, Yulia Tymoshenko. A spokeswoman said that Mr Barroso had told them that it was "unacceptable that the EU’s gas supply security is being taken hostage to negotiations between Russia and Ukraine”.

Mirek Topolanek, the Czech Prime Minister, whose country holds the rotating presidency of the 27-nation bloc, threatened Moscow and Kiev with "severe intervention" unless they settled a dispute that has lead to gas supplies to at least a dozen EU member states being cut off.

He did not say what action the EU could take against either country and admitted: "Obviously there's a certain political dimension."

Russia provides for about a quarter of Europe's gas needs, 80 per cent of which passes through pipelines in Ukraine. In a repeat of a similar stand-off three years ago, Russia started reducing its supplies to Ukraine on New Year's Day, complaining of unpaid bills and accusing Kiev of "stealing" gas destined for Western Europe.

Supplies through to Europe have been cut back gradually since then and came to a complete halt today.

At least 15,000 households in Bulgaria — which gets 92 per cent of its gas via the Ukrainian pipelines — found their heating cut off overnight and others were urged to reduce their consumption.

The International Energy Agency (IEA) said that that Bulgaria, Romania, Turkey and Greece would all start to experience "difficulties" if a cold spell in Southern and Eastern Europe continued into next week. Other countries affected include Austria, Slovakia, the Czech Republic, Bosnia, Croatia, Hungary, Macedonia and Serbia.

“Supplies have now come to a complete halt,” the IEA said in a statement. “This is completely unacceptable, given that European customers are not a party in this dispute."

The agency, which is the energy-monitoring and policy arm of the Organisation for Economic Co-operation and Development, said that it was “very concerned about the escalation of the dispute” between Gazprom, the Russian gas monopoly, and the Ukrainian Naftogaz.

“Despite the reassuring statements late December by both parties that supply to Europe would not be interrupted, supplies have now come to a complete halt,” it said. “The interruption is creating hardship during the coldest weather Europe has faced within a number of years."

In an interview with Reuters, the head of Naftogaz signalled a hard line in tomorrow's resumed negotiations. Oleh Dubyna said that he would insist on that Ukraine pay no more than $201 per 1,000 cubic metres of Russian gas for 2009, less than half the amount demanded by Russia.

Mr Dubyna dismissed as "absurd" Russian claims that Ukraine had shut down export pipelines to European customers, saying that all gas taps were on Russian territory.

The situation may soon become clearer. The Ukrainian Government said that Ms Tymoshenko had agreed to allow EU technical observers into Ukraine “immediately” to monitor gas supplies in her telephone conversation with Mr Barroso.

While the EU has repeatedly refused to referee what it sees as a commercial dispute between Moscow and Kiev, it looks increasingly likely that it will have to unless tomorrow's meeting produces a breakthrough.

“The EU has been reluctant to get involved into the negotiations between Russia and Ukraine, but soon it may have no other option,” said Svitlana Maslova and Koon Chow, analysts at the investment bank Barclays Capital. “In our view, the EU’s more active engagement in the conflict would help to resolve the dispute, as it did in 2006.”

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